Call Haggerty. They will be able to at least give you some references to get a good estimator out there. Of course, you should have already called them to figure out how much insurance it'll be. If not Haggerty then one of the other collector/antique car insurance companies. Don't trust something that you need an appraisal before buying to a company like Geico or Safe Auto.

If I knew what kind of car we were talking about I could give some better answers, but with such vagueness, I can only give you a general suggestion.